Surface water nitrate (NO3−) pollution from agricultural production and other sources is a well-established problem in the United States. For example, nitrate levels in the Mississippi River have more than doubled since 1965 and have increased 3 to 10-fold in the northeastern U.S. since 1990 (Vitousek et al, 1997). The widespread nitrate contamination of waterways across the country has led to significant environmental and human health challenges. In particular, the discovery of large anoxic dead zones in the Gulf of Mexico and the drop of fish populations in the Chesapeake Bay have placed the role of nutrient run-off from our agricultural industry in the public eye. Indeed, contamination in the Gulf of Mexico can be at least partially attributed to agricultural run-off in the Mississippi River Basin and, especially Midwest agricultural areas (USGS, DO1, 2000). It has been found that nitrate loss from agricultural land managed by subsurface or tile drainage systems is particularly high (Dinner, et al., 2002). The common use of these systems in the Midwest states correlates to elevated nitrate runoff. By EPA estimates, nitrate concentrations of up to 40 mg/L exist in and exit tile drainage systems into surface water (Sawyer, et al., 2006). It was also estimated in a 1997 east-central Illinois study that 49% of the inorganic nitrogen pool in agricultural soils was leached to tile drains and 30 exported to streams (David et al., 1997).
Development of efficient and economical tools and methods to mitigate agricultural and other nitrate runoff is therefore imperative. Besides the concerns of regulators, farmers themselves have an interest in managing nitrate concentrations in soils and surface water, as increasing numbers move towards sustainable agriculture and precision application of nutrients for resource efficiency and economic benefit. As the U.S. and other industrialized countries attempt to move towards sustainable economies and deal with existing environmental challenges, there is increasing focus on monitoring nutrients, particularly nitrates, in surface water produced at non-point sources such as agricultural operations and water treatment plants, as well across watersheds more generally.
There are two main technologies currently being used for nitrate sensing, including spectrophotometric devices (UV) and ion-selective electrodes (ISE). These technologies can be utilized in ecological and industrial environments with success. However, there are a number of disadvantages which are halting their widespread utilization, such as generally high cost and short lifetime or labor intensive maintenance, with some specific issues regarding sensing range and precision.
UV sensors are the industry standard for continuous monitoring. They operate by scanning for dissolved nitrate molecules using a 210 nm wavelength light and have a 95% confidence interval for a thirty second scan of 0.2 uM (Johnson and Coletti, 2002). Thus, these systems are typically used for deployments which require high sensing precision with minimal maintenance schedules. They have been used for long-term monitoring in drinking water, freshwater, watershed, marine, and wastewater applications where they are deployed for their high sensitivity (Hach, 2009). There are a variety of UV sensor systems available, along with customized packages, such as sondes, in which they can be installed, but the common characteristic among all of them is that they are capital-intensive devices: ranging from $5,000 (est.) to $15,000 (Hach NITRAX sensors). This cost is due to their complicated design and features (self-calibration, auto-cleaning, pressure-resistant casings, spectrophotometer equipment) which makes them difficult and expensive to fix if broken.
ISE sensors cost less than the UV sensors; at less than $500 (NexSense WQSensors' Nitrate ISE sensor, Hach ISE sensor). However, they have several drawbacks which make them less applicable in critical environments. Their main limitation is sensitivity to temperature and a severe interference by ionic compounds. This leads to one of the most limiting factors for widespread use: their poor performance in marine environments. Since ISEs need to be calibrated often and readings quickly become a function of other ionic compounds' concentrations, temperature, and age of electrode tip, they are typically used in controlled environments such as laboratories or as sensors for site visits (NexSense, 2010) rather than for long-term deployments. Additionally, the electrode tip ($75-$150) needs to be replaced twice a year, quickly adding to over 30% of the initial capital used for upkeep within the first year.
As described, existing methods for monitoring nitrate levels in surface water through periodic field monitoring and sample are labor and materials intensive, expensive (between $500 to $15,000 dollars per sensor, depending on the technology) and short-lived (average time to replace ˜6 month, depending on the technology). Moreover, given the mobility of nitrogen, the current monitoring practices give an incomplete picture of ongoing nitrate loss. As such, there is a need for improved, low-cost sensors for surface water monitoring.